<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <form id="f1">
        <input type="text" id="username"/><br/>
        <input type="password" id="pwd"/><br/>
        <input type="submit" value="注册" id="sub"/>
    </form>
</body>
<script>
    var f1 = document.getElementById("f1");
    var username = document.getElementById("username");
    var pwd = document.getElementById("pwd");
    var sub = document.getElementById("sub");
    username.onblur = function(){
        var val = username.value;
        //var val = this.value;
        if(val.length < 3){
            alert("用户名过短");
        }

    }
    // pwd.onfocus = function(){
    //     alert("请输入密码");
    // }
    f1.onsubmit = function(){
        alert("表单提交")
    }

</script>
</html>